1. Field of the Invention
The invention relates to perforating guns employed for perforating the casing and adjoining production formation of a subterranean well, and to methods for assembling and arming such gun in a manner that minimizes the risk of injury to the persons doing the assembly.
2. History of the Prior Art
The utilization of a plurality of vertically and angularly spaced shaped charges to effect the perforation of a subterranean well represents the modern approach to achieving perforation of the casing and the adjoining production formation. Such shaped charges are substantially concurrently fired by the ignition of a primer cord which passes successively past the primer ends of each of the shaped charge containers. It is not uncommon for the zone to be perforated to extend from ten to one thousand feet in length, thus necessitating the fabrication of the perforating gun as a plurality of axially stacked, substantially identical gun sections. The transmission of the firing energy from the uppermost section to the lowermost section is commonly accomplished through the mounting of booster charges on each of the ends of a primer cord which is utilized only within an individual gun section. The booster charges are required to insure that sufficient energy is transmitted from one primer cord to the next to insure a successive ignition of all primer cords. There have been many instances of unsuccessful transfer of ignition energy from one gun section to the next, necessitating the removal of the apparatus from the well.
As is well known to those skilled in the art, the booster charges commonly employed contain lead azide, an explosive which is extremely sensitive and susceptible to detonation by a relatively low level impact or shock. It is sufficiently dangerous that the assembly of perforating guns containing lead azide boosters at the factory is prohibited, due to regulations preventing the transport of such guns containing lead azide boosters on the public highways or on maritime carriers. It thus becomes necessary to insert the lead azide boosters in the perforating gun at the well site and, in this environment, it is readily possible to prematurely detonate the lead azide booster charges with the consequent high risk of injury to personnel effecting the assembly and arming of the multisection perforating gun.
In contrast, the primer cord commonly employed in perforating guns to effect the ignition of the shaped charges is relatively insensitive to inadvertent shock detonation and requires a substantial degree of heat to effect its ignition. Thus, a perforating gun construction which permits the firing of all of the shaped charges, regardless of the number of gun sections, by a single continuous length of primer cord, would obviously represent a substantial advantage for the industry.